This invention relates to an improved process for the indirect electrolytic oxidation of elemental phosphorus to phosphorous acid, (HO).sub.2 HPO.
The indirect electrolytic oxidation of elemental phosphorus to phosphorous acid is taught in copending application Case No. C-07-21-0308, simultaneously filed to Michael J. Dolan, which application is assigned to the same assignee as is the present case.
Phosphorous acid is available commercially as 30 percent and 70 percent aqueous solutions. The conventional method of preparation of phosphorous acid comprises hydrolyzing phosphorus trichloride according to the diagrammatically simplified reaction: EQU PCl.sub.3 + 3H.sub. 2 O .fwdarw. (HO).sub.2 HPO + 3HCl (1)
and evaporating the excess water and the hydrogen chloride which are formed. However, the methods involving these raw materials suffer from a number of disadvantages, most of which are inherent in the prior art methods.
One of the more obvious and vexing difficulties associated with the known methods of preparation of phosphorous acid is the absence of simple, effective, and efficient means of disposal for the large volume of hydrogen halide produced during the conversion (either hydrolysis or otherwise) of phosphorus trihalide to the desired acid. Means which have been proposed for this purpose are generally expensive and less than satisfactory.
As a result of the difficulties and disadvantages associated with the known methods of preparation, phosphorous acid remains a relatively expensive chemical compound.
It has now been discovered that the difficulties and disadvantages of the prior art methods are overcome by the process of the present invention which represents a substantial improvement in the sense that:
(a) the hydrogen halide generated is disposed of in situ in a manner which facilitates it being recycled for continued use, with the result that only a catalytic amount of hydrogen halide is required to be added initially; and PA1 (b) phosphorous acid is obtained relatively inexpensively with a resulting decrease in the commercial price of this important chemical compound.
A further advantage of the present invention is the ready availability of the essential reactants. The essential reactants are elemental phosphorus, hydrogen halide (which is reusable), water, and electric current. Moreover, the possibility of undesirable side reactions occurring is significantly reduced by conducting the electrolysis in an electrolysis medium containing in addition to elemental phosphorus and an aqueous solution of hydrogen halide, a non-aqueous solvent capable of dissolving the molecular halogen generated during the electrolysis as well as at least sufficient amounts of the elemental phosphorus to permit the oxidative reaction between it and the molecular halogen to proceed at a reasonable rate. As the desired reaction proceeds additional elemental phosphorus dissolves, thereby maintaining a continuous supply of dissolved elemental phosphorus available for reaction so long as some undissolved elemental phosphorus remains.
Various other advantages of this invention will become apparent from the accompanying description and claims.